Process for forming nonwoven web



April 14,- 1970 M. TALBERT 3,506,744

PROCESS FOR FORMING NONWOVEN WEB Filed NOV. 25, 1968 H s. 1 F|G.2

INVENT OR LARRY I. TALBERT ATTORNEY United States Patent 3,506,744 PROCESS FOR FORMING NONWOVEN WEB Larry Melvin Talbert, Madison, Tenn., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Continuation-impart of abandoned applications Ser. No. 554,100, May 31, 1966, and Ser. No. 633,655, Mar. 30, 1967. This application Nov. 25, 1968, Ser. No. 778,738

Int. Cl. B29c 24/00 US. Cl. 264-22 5 Claims ABSTRACT OF THE DISCLOSURE The positive triboelectric charge level of polyester fibers is enhanced by the presence therein of small amounts of alkali metal salts.

BACKGROUND OF THE INVENTION This application is a continuation-impart of my copending applications Ser. No. 633,655, filed Mar. 30, 1967 and Ser. No. 554,100, filed May 31, 1966 and now abandoned. This invention relates to an improved process for placing a stable, electrostatic charge on synthetic fibers and more particularly to an improved process for placing a stable, high level, positive electrostatic charge on moving polyester fibers by triboelectric charging or by a combination of triboelectric charging and corona charging.

Electrostatic charging of fibers is known in the art and is used for a variety of purposes including fiber dispersion, fiber deflection and fiber pinning to a grounded object. In the preparation of nonwoven fabrics from continuous filament fibers by the recently developed process described in US. Patent 3,338,992, a multifilament strand of continuous filaments under tension is electrostatically charged by known techniques, for example, by triboelectric charging or by passing the filaments through a corona discharge zone; the charged filaments are then forwarded by means of a jet device toward a web laydown zone; the tension on the filaments is released shortly after the exit of the jet device thereby permitting them to separate due to the repelling effect of the applied electrostatic charge; and the filaments, while thus separated, are collected as a nonwoven web. The process of this invention is particularly suited for use in the preparation of nonwoven fabrics by the aforementioned process.

Electrostatic charging of fibers can be accomplished by corona discharge, by the specific resistivity method described in the aforementioned patent, by triboelectric charging induced by contact with a suitable threadline guide or by other suitable means. The specific resistivity method is not generally useful with unmodified fibers other than polyamides since these do not normally have high enough electrical conductivity in the molten state. Triboelectric and especially corona charging have been most useful as means of imparting an electrostatic charge to synthetic fibers.

The preparation of nonwoven webs having randomly disposed polyester filaments that are separated from each other except at crossover points in accordance with the aforementioned process requires that the filaments be adequately dispersed prior to laydown. An electrostatic charge of sufiicient magnitude must be imparted to the filaments in order for the filament surface charge level, upon release of applied tension and the start of essentially free travel to the receiving surface, to be adequate to achieve this filament dispersion. Other things being equal, higher filament surface charge levels result in' nonwovens having greater randomness of filament disposition and better separation of the individual filaments in the fabric.

The term polyester as used herein comprehends a substantially linear polymer of fiber forming molecular weight comprising a series of predominately carbon atom chains joined by recurring carbonyloxy radicals The term polyester is intended to include copolyesters, terpolyesters and the like. Included for example are polyesters disclosed in Whinfield et al. US. Patent 2,465,319 and Kibler et al. US. Patent 2,901,466.

The preferred charge level cannot be obtained by triboelectric means alone with many polyester fibers nor can it be obtained by a combination of triboelectric and positive corona charging. The reason for failure to achieve a satisfactory triboelectric charge level is that the positive charging ability of the threadline guides or bars decays rapidly after initial start-up and thereafter produces only a low level charge. The amount of positive corona charging possible is limited by arcing between the electrodes and target bars which occurs at current levels too low to provide the required charge level.

Negative corona charging has heretofore been the preferred method for achieving the high charge levels required for adequate filament dispersion of synthetic fibers when preparation of an isotropic nonwoven sheet was desired. This is possible because of the greater current levels possible without arcing when negative corona charging is used. Although satisfactory electrostatic charge levels can be obtained by negative corona charging, this method is subject to certain deficiencies. Passage of the spinning polymer threadline through the guides required to control the yarn direction usually creates a positive triboelectric charge on the fibers, The magnitude of this charge is partly dependent on the amount of friction in the system and partly on the amount of decay of threadline guide or bar charging ability which occurs with time. These factors produce a variable positive electrostatic charge on the fibers which must be sufficiently overcome by negative corona charging to provide a resultant high level of negative charge. Variation of the positive triboelectric charge on the fibers results in variation of the net negative electrostatic charge and consequent nonuniformity of product. A further deficiency of negative corona charging is that nonconducting deposits build up on the target 'bar of the corona charging device resulting in decreased charge on the fibers with time and necessitating frequent cleaning of the target bar in order to maintain adequate charge levels.

SUMMARY OF THE INVENTION This invention provides a process for placing a stable, high level, positive electrostatic charge on a plurality of synthetic fibers, moving as a group such as a bundle or tow (substantially untwisted and unentangled), by triboelectric charging or by a combination of triboelectric charging and positive corona charging. This process was made possible by the discovery that small amounts of alkali metal salts in polyester fibers greatly enhance the positive triboelectric charge level. The positive triboelectric charge level can be further enhanced when the alkali metal salt is present in suitable amounts by using suitable metallic threadline guides which provide increasing charge with increasing rubbing contact. This effect of alkali metal salts is completely unexpected since ordinarily the presence of ionic material in a polymer greatly facilitates the decay of an electrostatic charge (see Shashoua, V. B. Journal of Polymer Science, 1, 169 [1963]). This particularly true of polyelectrolytes. Uniformly high, positive, tribo electric charges are obtained over extended periods of time using the process of this invention.

In accordance with the present invention, it has been found that triboelectric charge receptivity and retention of guide charging ability of a moving bundle or tow of polyester filaments is increased if there is incoporated in said filaments an amount of alkali metal salt sufficient to provide at least 0.02 gram atoms of the alkali metal component of the salt per 100 lbs. of filament. The alkali metal salt may be incorporated in any of several ways. One method involves adding an amount of an alkali metal acetate to the polymer precursors and then proceeding with polymerization. If desired the alkali metal acetate may be blended thoroughly with the polymer itself prior to spinning. Still another method is exemplified by Example V below.

BRIEF DESCRIPTION OF THE DRAWINGS This invention can be better described by referring to the following figures:

FIGURE 1 is a schematic representation of one process wherein the present invention has applicability.

FIG. 2 represents a variation of FIGURE 1.

Referring to FIGURE 1, molten polymer containing an amount of alkali metal salt sufficient to provide more than about .02. gramatoms of the alkali metal component of the salt per 100 lbs. of polymer is extruded through spinneret 1 and the resulting threadline is passed through threadline guides 2, 3, and 4 and then passed over the target bar of two suitable corona charging means 5, 6 in succession, through an additional threadline guide 7 to feed roll 8 then to draw roll 9 and through laydown jet 10-to a suitable collecting means. The feed roll 8 and draw roll 9 may be operated at the same peripheral speed or the draw roll 9 may be operated at a higher peripheral speed than the feed roll 8 to provide increased orientation of the fiber structure.

Referring to FIGURE 2, molten polymer containing an amount of alkali metal salt sufficient to provide more than about .02 gram atoms of the alkali metal component of the salt per 100 lbs. of polymer is extruded through spinneret 11, through threadline guides 12, 13 and 14, over the target bar of a corona charging means 15, through additional threadline guides 16, 17 and 18, over the target bars of two successive corona charging means 19 and 20, through an additional threadline guide 21 to feed roll 22 and draw roll 23 and finally through laydown jet 24 to a suitable collecting means. The feed roll 22 and draw roll 23 may be operated at the same peripheral speed or the draw roll 23 may be operated at a higher peripheral speed than the draw roll 22 to provide increased orientation of the fiber structure.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the process of this invention the alkali metal salt should be present in a concentration sufficient to provide at least 0.02 gram atoms of alkali metal component of the salt per 100 lbs. of fiber to give improved charge receptivity and retention of guide charging ability. The preferred alkali metal component concentration is about 0.1 gram atoms per 100 lbs. of fiber. Concentration above 0.1 gram atoms per 100' lbs. of fiber may provide some further improvement. Alkali metal salts are readily available, low in cost, and relatively free from interfering reactions with other materials.

In general, the fibers useful in the process of this invention are comprised of polymers, preferably synthetic organic polymers, which are low in the triboelectric series; that is, they have a lesser tendency to assume a positive triboelectric charge (see Shashoua, V. E., Journal of Polymer Science, 1, 169 [1963]). The preferred fibers of this invention are polyesters, particular ly fibers of polyethylene terephthalate or polyethylene terephthalate copolymers.

The alkali metal salt containing fibers used in the process of this invention may contain minor amounts of additives in addition to these salts to provide other advantages so long as these do not detract appreciably from the positive electrostatic charge level or stability of the resulting fibers. Preferred fibers for carrying out the process of this invention are comprised of polyethylene terephthalate containing 0.02 gram atoms or more alkali metal per lbs. of fiber added in the form of an alkali metal salt. The polyethylene terephthalate preferably has a relatively viscosity of 20-60 as determined at 30 C. using an 8.04% by weight solution of the polymer in a solvent consisting of 10 parts by volume of phenol and 7 parts by volume of trichlorophenol.

The process of this invention may also be used when fibers of different composition are spun at the same time. By different composition is meant any two materials which when passed through a common corona charging field collect charges at different rates as measured in microcoulombs/meter In other words, the differences may be chemical differences such as when one has two different polymeric structures. On the other hand, the differences may be physical such as would be encountered with filaments of different cross-section, different denier, or different surface characteristics.

When different compositions are used, it may be desirable to provide different amounts of alkali metal salt in the different compositions so as to obtain fibers with similar levels of electrostatic charge. Alternatively, the two fibers may be handled separately and the charge level on the two different fibers equalized by varying the threadline guide scheme so as to provide different levels of triboelectric charging on the two fibers. The fibers may then be combined and charged in a suitable corona charging device or they may be charged with separate corona charging devices and combined at the feed roll. Other modifications of the charging scheme will be obvious to one skilled in the art.

A preferred method for effecting bonding of a nonwoven web, thereby converting it to a strong nonwoven fabric, is by means of binder filaments distributed uniformly throughout the nonwoven web of matrix filaments. The binder filaments have a lower softening temperature or melting point than the matrix filament constituting essentially the remainder of the filaments of the web. Upon subsequent heating of the nonwoven web, the binder filaments adhere to the matrix filament while retaining their filamentary character, or they may melt and flow to the matrix filament crossing points and form granule bonds upon cooling. A preferred combination of matrix and binder fibers to be used in the process of this invention is polyethylene terephthalate as the matrix fiber and the copolymer of ethylene glycol and 79% terephthalic acid and 21% isophthalic acid as the binder fiber. The process of this invention provides uniform distribution of binder fibers throughout the web. Other polyesters and copolyesters are also suitable for use in the process of this invention.

When it is desired to use a corona charging device in the process of this invention, one such suitable device is the corona discharge means described in Di Sabato and Owens, US. Patent 3,163,753. The charge on the filaments can be measured by collecting them for a given period of time in a pail coulombmeter. The coulombmeter is placed at the exit of the jet and relaxed fibers from the jet are collected, interrupting for a moment the flow of fibers to the collecting means. The collected sample is Weighed and the average denier of the filaments is determined. From these data and the density of the fibers, the charge in microcoulombs/meter of filament surface can be determined.

While the process of this invention may be advantageously used in a process such as that described in US. Patent 3,338,992 for preparing nonwoven webs, other uses will be obvious to One skilled in the art.

The following examples will serve to illustrate the process of this invention but are not intended to be limitative in any way.

Example I Polyethylene terephthalate containing 0.1 gram atoms of sodium per 100 lbs. of polymer (added as sodium acetate during the polymerization) is spun through a 250-hole spinneret using the yarn handling system described in FIGURE 1. The feed and draw roll peripheral speeds are 3200 y.p.m. Threadline guides 2, 3, 4 and 7 are alumina coated. The corona charging devices used are similar to those described in Di Sabato and Owens, US. Patent 3,163,753.

(A) The system is operated with the corona charging devices turned off and a minimal amount of snub. The yarn at the laydown jet has a positive electrostatic charge of 8.5 microcoulombs/meter (B) The system is operated as in (A), above, except about /3 the possible snub is used. The yarn at the laydown jet has a positive electrostatic charge of 8.5 microco ulombs/meter (C) The system is operated as in (A), above, except the maximum possible amount of snub is used. The yarn at the laydown jet has a positive electrostatic charge of 8.6 microcoulombs/meter (D) The system is operated with positive corona charging at 18,000 volts and 0.3 ma. The yarn at the laydown jet has a positive electrostatic charge of 26.5 microcoulombs/meter (E) The system is operated with negative corona charging at 0.75 ma. The yarn at the laydown jet has a negative electrostatic charge of 12.9 microcoulombs/ meter Example II Polyethylene terephthalate containing 0.1 gram atom of sodium per 100 lbs. of polymer is spun as in Example I except that the alumina coated guides 2 and 3 are replaced with guides having a matte chrome surface.

(A) The system is operated with the corona charging devices turned off and a minimal amount of snub. The yarn at the laydown jet has a positive electrostatic charge of 10. 8 microcoulombs/meter (B) The system is operated as in (A), above, except about /3 the possible snub is used. The yarn at the laydown jet has a positive electrostatic charge of 13.3 microcoulombslmeter (C) The system is operated as in (A), above, except that about /3 of the possible snub is used. The yarn at the laydown jet has a positive electrostatic charge of 15.5 microcoulombs/metefl.

(D) The system is operated as in (A), above, except that the maximum possible snub is used. The yarn at the laydown jet has a positive electrostatic charge of 17.0 microcoulombs/meter (E) The system is operated as in (A), above, using maximum possible snub except the alumina coated threadline guide 7 is replaced with a matte chrome covered guide. The yarn at the laydown jet has a positive electrostatic charge of 30 microcoulombs/meter (F) The system is operated as in (A), above, except positive corona charging at 18,000 volts and 0.3 ma. is used. The yarn at the laydown jet has a positive electrostatic charge of 30.5 microcoulombs/meter Example III Polyethylene terephthalate containing no added sodium salt is spun through a 250-hole spinneret using the yarn handling system described in FIGURE 1. The feed roll and draw roll peripheral speed is 3200 y.p.m. for (A) and (C) and 3730 y.p.m. for (B). Threadline guides 2, 3, 4 and 7 are alumina coated. The corona charging devices are similar to those described in Di Sabato and Owens, US. Patent 3,163,753.

(A) The system is operated with the corona charging The copolyester polyethylene terephthalate/isophthalate (79/21) containing 0.1 gram atoms of sodium per 100 lbs. of polymer (added as sodium acetate during the polymerization) is spun through a 50-hole spinneret using the yarn handling system described in FIGURE 2 except that guides 14 and 16 were omitted. The feed and draw roll peripheral speed is 3200 y.p.m. Guides 12, 13 and 17 had matte chrome surfaces while guides 18 and 21 were alumina coated. The corona charging devices used are similar to those described in Di Sabato and Owens, US. 3,163,753.

(A) The system is operated with all corona charging devices turned off and moderate snub. The yarn at the laydown jet has a positive electrostatic charge of 24.5 microcoulombs/rneter (B) The system is operated as in (A), above, except corona charging device 15 is used to apply a positive electrostatic charge at 14,000 volts and 0.2 ma. The yarn at the laydown jet has a positive electrostatic charge of 3 7.4 microcoulombs/meter Example V A copolyester prepared from ethylene glycol and 98% dimethyl terephthalate and 2% of the sodium salt of dimethyl 5-sulfoisophthalate (3.1 gram atoms of sodium per 100 lbs. of polymer) is spun through a 20-hole spinneret using the yarn handling system described in FIG- URE 2 except threadline guides 14, 17 and 18 are eliminated and an additional threadline guide 21.5 is placed between guide 21 and the feed roll 22. Threadline guides 12, 13 and 21 and the target bars of the corona charging devices 15, 19 and 20 have matte chrome surfaces. Threadline guides 16 and 21.5 are alumina coated. The feed roll 22 and the draw roll 23 are operated at a peripheral speed of 3200 y.p.m.

(A) With the corona charging devices turned off and the maximum possible amount of snub, the yarn at the laydown jet has a positive electrostatic charge of 38 microcoulombs/meter Example VI Polyethylene terephthalate containing 0.1 gram atoms of sodium per 100 lbs. of polymer (introduced as sodium acetate) is processed as in (A) of Example V. The yarn at the laydown jet has a positive electrostatic charge of 28 microcoulo1nbs/meter Example VII A copolymer prepared from ethylene glycol and dimethyl terephthalate and 5% of the sodium salt of dimethyl 5-sulfoisophthalate (7.7 gram atoms of sodium per lbs. of polymer) is processed as in (A) of Example V. The yarn at the laydown jet has a positive electrostatic charge of 39 microcoulombs/meter Example VIII Polyethylene terephthalate containing 0.1 gram atoms of lithium per 100 lbs. of polymer (added as lithium acetate during the polymerization) is spun through a 20-hole spinneret using the yarn handling system described in FIGURE 2 except the corona charging device 15 is replaced with a threadline guide, threadline guides 16 and 17 are removed and two threadline guides are placed between threadline guide 21 and the feed roll. All threadline guides are chromium plated. The feed and draw roll peripheral speeds are 3200 y.p.m. The corona charging devices were turned off. Using the maximum possible amount of snub, the yarn at the laydown jet has positive electrostatic charge of 20.3 microcoulombs/meter A control yarn containing 0.1 gram atoms of sodium per 100 lbs. polymer also had a positive electrostatic charge of 20.3 microcoulombs/metefl.

Example IX Polyethylene terephthalate containing 0.1 gram atoms of potassium per 100 lbs. of polymer (added as potassium acetate during the polymerization) is spun as in Example VIII. Using the maximum possible amount of snub, the yarn at the laydown jet had a positive electrostatic charge of 19.7 microcoulornbs/meter? A control yarn containing 0.1 gram atoms of sodium per 100 lbs. of polymer had a positive electrostatic charge of 19.8 mierocoulombs/metefl.

Example X Polyethylene terephthalate/isophthalate (79/21) containing 0.1 gram atoms of cesium (added by dry blending cesium sulfate with the polymer during flake drying) is spun as in Example VIII. Using the maximum possible amount of snub, the yarn at the laydown jet had a positive electrostatic charge of 22.4 microcoulombs/meter A polyethylene terephthalate/isophthalate (79/21) control yarn containing no cesium salt had a positive electrostatic charge of 6.6 microcoulombs/meter What is claimed is:

1. In a process wherein a plurality of polyester filaments traveling as a group under tension are triboelectrically charged by rubbing contact with a surface, and subsequently are forwarded by means of a jet device toward a web laydown Zone, the tension on the filaments being released at the exit of the jet device thereby permitting them to separate ,due to the repelling effect of the applied electrostatic charge, and the filaments while thus separated being collected as a nonwoven web, the improvement comprising incorporating in said filaments prior to said rubbing contact, an amount of alkali metal salt sufiicient to provide at least 0.02 gram atoms of the alkali metal component of the salt per lbs. of filament.

2. The process of claim 1 wherein a sodium salt is incorporated in the filaments.

3. The process of claim 2 wherein the sodium salt is sodium acetate.

4. The process of claim 1 wherein the filaments are charged by corona charging in addition to triboelectric charging prior to entry into the jet device.

5. The process of claim 1 wherein the polyester filaments are polyethylene terephthalate.

References Cited UNITED STATES PATENTS 2,074,285 3/1937 Studt et al 264210 X 2,924,503 2/1960 Reese 264-211 X 2,927,841 3/1960 Ben 26421l X 3,156,752 11/1964 Cope 264-22 X 3,338,992 8/1967 Kinney 264-24 3,367,926 2/1968 Voeks 26093.5 3,402,227 9/ 1968 Knee 264-24 FOREIGN PATENTS 954,024 4/ 1964 Great Britain.

JULIUS FROME, Primary Examiner J H. WOO, Assistant Examiner US Cl. X.R. 

